During the distillation liquid is vaporized and the vapor is condensed. It is suitable for the separation of liquids having differing vapor pressure and for the full or partial separation of liquids from salt solutions.
A distillation apparatus which can be used, realized and is practicable must be both favorably priced and also energy-efficient. A distillation process is only expedient when both conditions are satisfied.
In classic thermal distillation processes such as the multi-effect relaxation vaporization or multi-stage relaxation vaporization (Multiple Stage Flash MSF) and in thermo-mechanical processes such as the vapor compression process (Vapor Compression VR) the liquid to be concentrated and the vapor are in one space and essentially at the same absolute pressure.
In the membrane distillation process the liquid to be concentrated is bounded at least at one side by a vapor permeable, liquid-tight membrane. This membrane wall is followed by a vapor space which is located at a lower vapor pressure than the liquid to be concentrated. Through the pressure difference vapor arises at the boundary surface of the liquid to be concentrated to the membrane and passes through the membrane.
In the known membrane distillation process the vapor is precipitated at the side of the membrane remote from the liquid in the adjacent colder condensate or the vapor is condensed at a colder surface in the vapor space or it is sucked off and condensed outside.
In the membrane distillation a porous, vapor permeable material is used. In U.S. Pat. No. 3,340,186 an apparatus is described which utilizes an air-filled, micro-porous, hydrophobic membrane. The method described here is based on a direct contact membrane distillation. The warm flow of sea water and the cold flow of distillate are in direct contact with the membrane.
In EP-A-0 088 315 an apparatus is described for the continuous distillation of a hot salt-containing solution or of liquid mixtures with differing vapor pressure.
This apparatus consists of a thermally conductive, vapor impermeable layer which forms an elongate wall, a hydrophobic, vapor permeable membrane which forms an adjacent or oppositely disposed wall and forms an elongate distillate collection chamber or passage together with the vapor impermeable layer. The chamber has an outlet for the distillate. For the preferred embodiment of this membrane distillation a spiral coil configuration is used. Cold sea water or feed flows in a spiral-shaped chamber into the middle and thereby takes up heat from the condensation surface. The feed preheated by the condensation process of the distillate is now further heated by a heating system and then led into the concentrate channel. The hot solution flows outwardly through the channel bounded by the membrane. On flowing through the concentrate channel a part of the solution vaporizes through the membrane.
In EP A 1 185 356 a process for the cleaning of a liquid by membrane distillation is described, in which vapor from a liquid flow arises and passes through a porous wall bounding the liquid flow. The vapor condenses at a cold condenser surface whereby a flow of condensate is formed. The condenser surface separates a supplied liquid flow from the distillate flow. This supplied liquid flow flows in counter-flow to the vapor yielding liquid flow. In order to enlarge the distillate flow a pressure is maintained in the gas channel which, however, lies below the environmental pressure, but above the vapor pressure of the liquid yielding the vapor.
There are now a series of problems which arise in connection with the known membrane distillation processes.
In all known membrane distillation processes in which a vapor space is used this vapor space is separated from the liquid to be vaporized by a vapor permeable, water-tight membrane. This leads in all known membrane distillation processes to a situation in which the vapor space and the space containing the liquid to be concentrated which is bounded by the vapor permeable, liquid-tight membrane are at different absolute pressures.
In membrane distillation processes in which the vapor space is located at a pressure in the region of the environmental pressure the solution is at a pressure level above the environmental pressure, with this pressure level being composed of by the static pressure and the hydraulic pressure loss. This leads to an absolute pressure difference between the space containing the liquid to be vaporized and the space containing the vapor which is separated by the vapor permeable, liquid-tight membrane.
In membrane distillation processes in which the vapor space is at a pressure level below the environmental pressure the absolute pressure difference between the space containing the liquid to be concentrated and the space containing the vapor which is separated by the vapor permeable, liquid-tight membrane is increased by the negative pressure.
This pressure difference between the space containing the liquid to be concentrated and the vapor space, in many applications also associated with a temperature loading of the vapor permeable, liquid-tight membrane by the heated liquid to be concentrated, leads to a high mechanical and thermal loading of the membrane. This substantially reduces the lifetime of the membrane which mainly consists of plastic.
A further problem results in membrane distillation processes in which the heat transfer essentially takes place by condensation and subsequent vaporization. If the heat transport namely takes place through the liquid space, which is bounded on one side by a vapor permeable, liquid-tight membrane and on the other side by a vapor-tight and liquid-tight condensation surface, essentially only by heat conduction in the liquid then, in relation to the condensation and vaporization, only a small quantity of heat can be transferred. The heat quantities comparable to the heat of condensation can only be transferred in the space containing the liquid to be concentrated when boiling can be achieved in this space.